1. Technical Field
This disclosure relates generally to implantable medical devices, and more particularly to methods, apparatus, and systems for assessing charge imbalance in implantable medical devices.
2. Background Information
There have been many improvements over the last several decades in medical treatments for disorders of the nervous system, such as epilepsy and other motor disorders, and abnormal neural discharge disorders. One of the more recently available treatments involves the application of an electrical signal to a patient's tissue to reduce various symptoms or effects of such neural disorders. For example, electrical signals have been successfully applied at strategic locations in the human body to provide various benefits, including reducing occurrences of seizures and/or improving or ameliorating other conditions. A particular example of such a treatment regimen involves applying an electrical signal to the vagus nerve of the human body to reduce or eliminate epileptic seizures, as described in U.S. Pat. No. 4,702,254 to Dr. Jacob Zabara, which is hereby incorporated by reference in its entirety in this specification. Electrical signals may be applied to the vagus nerve by implanting an electrical device underneath the skin of a patient and electrically stimulating tissue, organ(s) or nerves of the patient. The system may operate without a detection system if the patient has been diagnosed with epilepsy, periodically applying a prophylactic series of electrical pulses to the vagus (or other cranial) nerve intermittently throughout the day, or over another predetermined time interval. Alternatively, the system may include a detection system to detect one or more physiological parameters associated with a disorder (e.g., changes in brain activity as evidenced by EEG signals). When the physiological parameter is detected, the electrical signal is then applied to a target body location in response.
Typically, implantable medical devices (IMDs) involving the delivery of electrical signals to, or the sensing of electrical activity in, body tissues (e.g., pacemakers for sensing and applying a signal to heart tissue, and vagus nerve stimulators for similarly sensing or applying a signal to a vagus nerve) comprise a pulse generator for generating the electrical signal and a lead assembly coupled at its proximal end to the pulse generator terminals and at its distal end to one or more electrodes that interface with the body tissue to which the signal is applied. As used herein “stimulation” refers to the application of an electrical signal to a target body tissue, regardless of the effect that signal is intended to produce.
In providing a stimulation signal to a target body tissue, a continuous or net charge at the electrode/tissue interface is undesirable. Because stimulation involves applying an electrical charge to body tissue, IMDs are required to ensure that the net charge at the electrode/tissue interface is approximately zero, i.e., that the stimulation is charge balanced. IMD manufacturers use output coupling capacitors between the output circuits of the pulse generator and the electrodes to block errant continuous direct current (“DC”) and serve as “passive” charge balancing components for the electrical signals being applied to the tissue. Charge built up on the electrodes during stimulation is offset by use of these output coupling capacitors, and discharged when delivery of a portion of the electrical signal is completed—typically after delivery of an individual pulse in a pulsed electrical signal. A “discharge phase” may be observed for a period, for example, after a monophasic stimulation phase. The stimulation phase and the discharge phase taken together may be considered a charge-balanced pulse in a signal comprising a plurality of such pulses.
Additionally, some IMDs may employ additional “active” charge balancing to reduce (or eliminate altogether) the workload on the passive charge balancing components (i.e., the output coupling capacitors). For active charge balancing, a stimulation of opposite polarity is applied at the electrode/tissue interface in a second phase after the initial stimulation. In such IMDs, active stimulation is set “a priori” based upon the programmed stimulation therapy. For example, a 1 mA pulse of 500 μS would be actively charge balanced by an opposite-polarity pulse of equal charge (Q=I*T) such as 1 mA for 500 μS or 0.25 mA for 2 mS. Since active charge balancing schemes are setup “a priori,” and without examination and assessment of the actual amount of net charge remaining on the electrodes, active charge balancing units may not account for system problems.
In many IMDs that deliver an electrical signal, two electrodes (i.e., one cathode and one anode) are used to deliver the signal. However, some IMDs, such as pain neurostimulators, deliver electrical signals through multiple electrodes (e.g., 3 or more electrodes), and given the dynamic variation associated with delivery of the electrical signal and electrode switching to deliver that signal, the likelihood of residual charge at any electrode/tissue interface is increased in multi-electrode IMDs.
The present disclosure is directed to assessing when an undesirable net charge exists on at least one electrode, and preferably on each, electrode of an IMD, despite passive or active charge balancing efforts, such as when the output capacitors have failed, or the stimulation therapy being delivered is driven too hard for the particular output capacitors used.